Infrared Photodissociation Spectroscopy of C 4 N - , C 6 N - and C 8 N .

The gas-phase vibrational spectroscopy of cold C _2n N ^- (n=2-4) anions is investigated in the CC and CN multiple bond stretching region (1700-2250 cm ^-1 ) by means of infrared photodissociation (IRPD) spectroscopy in a cryogenically cooled ion trap of the corresponding messenger-tagged complexes. The IRPD spectra are assigned to N-terminated linear structures with triplet ground states ( ³ Σ ^- ) based on a comparison with harmonic vibrational frequencies and intensities from density functional theory computations. In contrast to the polyacetylenic C _2n+1 N ^- anions, the linear C-C chains investigated here exhibit cumulenic character, which is most pronounced in C ₄ N ^- and decreases with chain length. Additional intense transitions are observed for C ₆ N ^- above 3000 cm ^-1 and are attributed to overtone and combination bands involving the CC stretching modes, based on anharmonic computations. The influence of a D ₂ tag on the vibrational features of C _2n N ^- anions is shown to be small.
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